Toroid systems for focused neuromagnetic stimulation

As a contactless applicator, the toroidal coil was proposed some time ago for stimulation of neural tissues in order to replace the electrodes. However; as compared to electrical stimulators, conventional toroids exhibit severe disadvantages including their relatively large volumes, their low power efficiency, and their low spatial accuracy. Here, the authors propose and analyse a new approach for toroidal coil systems that help to overcome the above mentioned problems. The method consists in passing the nerve trunk through the holes of two parallel toroidal coils. Their driving currents are opposing each other. This results in induced electric fields along the axis of the two toroidal coils. Focusing peaks for the induced fields and their derivatives, the so-called activating functions for long straight nerves, can be obtained by this method in the center in between the two toroids. Based on mathematical derivations and a numerical analysis of the induced fields and the activating functions, the optimum spacing between the two toroids was found to be equivalent to their radius. This new toroidal stimulation system might be useful in clinical applications involving the stimulation of nerve bundles such as the sciatic nerve or the nerves in the pelvic region controlling the bladder.